1. Field of the Invention
The present invention relates to wireless communication systems, and more specifically, to turning on and off a Mobile Assisted Handoff (MAHO) feature in a mobile station during a call.
2. Description of the Related Art
Time Division Multiple Access (TDMA) is a known implementation of multiple communication channels by using different time slots in a shared frequency. Transmission from mobile stations typically occurs in bursts in the uplink direction, with only one mobile station transmitting to a base station at any given time. In the downlink direction, the base station is usually transmitting continuously, with the mobile stations listening only during their assigned time slot.
In TDMA cellular systems, a technique known as mobile assisted hand off (MAHO) is used. During a call, channel quality between a mobile station and a serving base station may degrade such that a handoff of the call to another base station becomes necessary. Conventional MAHO is a process in which mobile telephones assist in handoff decisions by sending radio channel quality information back to the serving base station. A serving base station is a base station handling the communication needs of mobile stations within a certain coverage area, sometimes referred to as a cell. A mobile station whose communication signals are being handled by a serving base station is referred to a served mobile station.
TDMA systems use two types of radio channel quality information: a received signal strength indicator (RSSI) of channels from surrounding base stations, and an estimated bit error rate (BER) of a served mobile station""s operating traffic channel. The bit error rate (BER) is estimated using the result of forward error correction codes for speech data and call processing messages. Having a served mobile station report quality information also allows for measurements of the uplink and downlink RF signal quality that are not possible from a serving base station. The served mobile station typically reports channel quality information on either of a Slow Associated Control Channel (SACCH) or a Fast Associated Control Channel (FACCH).
Using MAHO signal measurements from mobile stations, a serving base station of a cellular system can decide when a handoff may be necessary. The MAHO information from mobiles also reduces data traffic between adjacent base stations.
During the conventional MAHO process, a serving base station sends a served mobile station a Measurement Order message containing a list of radio channels corresponding to the setup channels/broadcast control channels (BCCH) from up to 12 neighbor nonserving base stations for nonserving cells (but usually only six nonserving neighboring cells are measured). During its idle time slots, the served mobile measures the channel quality of the channels on the list including the traffic channel it is currently operating on for its particular serving base station. The served mobile averages the channel quality measurements, then continuously sends MAHO channel quality reports back to its serving base station periodically over either an SACCH or FACCH. However, the serving base station may send a Stop Measurement Order message to terminate all channel quality measurements and reports within its coverage area. In response, every served mobile station ceases making channel quality measurements and reports.
A mobile switching center (MSC) combines the MAHO measurements with its own information such as knowing which cells have available idle voice or digital traffic channels to determine which radio channel will offer the best quality. The mobile switching center allocates the best channel when a call handoff is required.
MAHO as conventionally implemented is a feature that is constantly on in mobile stations to ensure continuous mobile wireless TDMA communication. However, processing all channel quality measurements taken during all mobile stations"" unassigned time slots causes a heavy computational load on the serving base station. Also, the SACCH and FACCH become polluted with channel quality measurement data from all mobile stations within a coverage area.
Conventionally, even mobile stations which are stationary, and whose signal quality is not changing, are constantly collecting and transmitting signal quality measurements. Hence, there exists a need for a process which will ensure continuous mobile wireless TDMA communication, but which will reduce the computational load on the serving base station, and free the SACCH and FACCH for other messages.
Accordingly, the present invention is directed to a method for selectively disabling a mobile assisted handoff (MAHO) feature that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
The present invention provides a methodology for selectively enabling and disabling MAHO in a TDMA wireless communication system on a per-mobile station basis based on a triggering signal received at a serving base station.
In one embodiment of the present invention, the triggering signal is originated at an input device at a base station or the MSC. The triggering signal specifies the mobile station or set of mobile stations for which the MAHO feature is to be disabled. The serving base station then transmits a MAHO disable signal uniquely identifying those mobile stations for which MAHO is to be disabled.
In another embodiment of the invention, the triggering signal is originated from a served mobile station and contains information from which a relative position of the served mobile station may be derived. If the served mobile station has not moved from a previous position, the serving base station transmits a MAHO disable signal. If the served mobile station has moved from a previous position, the serving base station transmits a MAHO enable signal. The position information in the triggering signal may indicate a signal propagation delay between the base and mobile station, or it may indicate coordinates of the mobile station.
In another embodiment of the invention, the served mobile station constantly transmits channel quality measurements as in conventional MAHO, in addition to the triggering signals from which position may be derived. If the serving base station determines that the served mobile station has not moved, the serving base station does not process the received channel quality information.